Now i noticed that GStreamer is detecting a mime type of "application/x-ms-dos-executable", which is definitely not right. When i run file -i, sure enough, i get the wrong mime type. Some files get "application/x-dosexec"; other files, "application/octetstream"

I wouldn't be surprised if these don't help you out in getting the file to play, but:

1) Don't use playbin with gstreamer-0.10, use playbin2
2) Use gstreamer 1.x instead in the first place (there playbin2 is moved to playbin and the old 0.10 playbin simply deleted, as backwards compatibility was broken anyways)._________________GNOME team lead; GStreamer

Is a non-working wavpack file shareable? I believe I have a couple samples somewhere and those used to work, so would be good to have something problematic in my testing set._________________GNOME team lead; GStreamer

I downloaded some test files from http://wavpack.gl.tter.org/. To my surprise, i can play them all. So i guess something is screwed up with my files themselves. Which is a little strange because foobar2000 in Windows can play them fine.

That's most unfortunate, but of course understandable.
Perhaps if you opened an actual bug report, either in Gentoo, or perhaps even upstream if you've tested with gstreamer 1.2.3, we could try to narrow the issue down with debugging instructions in a slow process, and get this figured out. It seems the "MIME" stuff seems of interest to narrow down at first. Even I might have some ideas on how to narrow this down, without access to the files, after some poking around in the relevant code, but for that I'd prefer a bugzilla ticket going on for the debugging exchanges._________________GNOME team lead; GStreamer